There can be such a thing as a singer who's not worth dealing with. I don't know if this situation really fits that bill, though. It sounds like you're feeling territorial, which is understandable, but don't get too stuck on it.
On an instrument, you can always play higher up the neck, but the singer has physical constraints to what notes they can hit, what songs work for their voice tonally, and for that matter how their voice may need to warm up into the gig or how they need to pace it to last the full night. I've sung lead (badly) and strained my throat halfway through a gig, and the rest of the night was... not pretty. So stuff like song selection and set list, as others have said, you do want the input of all band members to count, but it's inevitable that the singer is going to be in the driver's seat there.
Likewise, no matter how much everyone else contributes, the rest of the world will see the singer as the front person that they identify the band by. I've been very happy with most of the singers I've played with over the years and always felt I had a say, but Wipt was Jeannie's band, Mugshot Monday is Nikki's (even though I'm the one who came up with the name!), etc. Six Cowards was MY band... because I did 2/3 of the singing (and other reasons, but that was a necessary piece of it).
I guess another way to look at it would be, what are the prospects of finding another available singer in your market who would meet your needs for a singer WITHOUT triggering these tensions? Unless you're going to be an instrumental act, you'll need someone to sing, and it doesn't sound like you've had your pick of the litter. Sometimes it's a matter of working with the reality of what you've got.
